In an internal combustion engine for an automobile, various types of feedback control are performed. In the case of a diesel engine, for example, feedback control is used during boost pressure control, exhaust gas recirculation (EGR) control, and air-fuel ratio control.
PI control (proportional integral control) is a typical feedback control method used in an internal combustion engine. To control the air-fuel ratio of a diesel engine, for example, a feedback control system having a control logic such as that shown in a block diagram in FIG. 6 is used. In this feedback control system, a target air-fuel ratio is determined in relation to the air-fuel ratio, which serves as a control amount. A measured air-fuel ratio is then obtained from an air-fuel ratio sensor, whereupon an air-fuel ratio deviation, which is a deviation between the target air-fuel ratio and the measured air-fuel ratio, is calculated. A P term (a proportional term) is then calculated by multiplying the air-fuel ratio deviation by a predetermined proportional gain Gp. Further, an I term (an integral term) is updated by adding a value obtained by multiplying the air-fuel ratio deviation by a predetermined integral gain Gp to a previous value of the I term. The P term and the I term are then added together, whereupon correction amounts to be applied to a fuel injection amount and an air amount serving as operation amounts are calculated on the basis of a PI term (a proportional integral term) constituted by the sum of the P term and the I term. Note, however, that in the control logic of FIG. 6, a magnitude of the PI term is restricted by a guard. A value of the guard is a variable value that is modified in accordance with an operating condition of the engine.
Restricting the magnitude of the PI term using a guard is a conventional technique disclosed in Japanese Patent Application Publication No. 2008-291752 (JP 2008-291752), for example. The correction amount relating to the operation amount of the internal combustion engine is calculated on the basis of the PI term, and therefore, when the guard is not provided in relation to the PI term, various problems occur during an operation of the internal combustion engine.
Taking control of an air-fuel ratio of a diesel engine as an example, when the operating condition of the engine varies rapidly, the air-fuel ratio deviation may increase rapidly, causing the value of the PI term to increase or decrease rapidly. When the rapidly increased or rapidly reduced value of the PI term is used as is to calculate the correction amount applied to the fuel injection amount or the air amount, an excessive correction amount may be applied to the engine, and as a result, the operating condition of the engine may become unstable.
Further, when an abnormality occurs in the air-fuel ratio sensor, for example when an output value of the air-fuel ratio sensor becomes stuck at a certain value, the air-fuel ratio deviation may never be eliminated, and as a result, the PI term may continue to increase or decrease. When the continuously increasing or decreasing value of the PI term is used as is to calculate the correction amount applied to the fuel injection amount or the air amount, an actual air-fuel ratio may gradually diverge from the target air-fuel ratio.
The reason for providing the guard in relation to the PI term is to prevent such problems from occurring. Taking the control logic shown in FIG. 6 as an example, when the value of the PI term exceeds the value of the guard, the value of the guard is applied to the engine instead of the value of the PI term. In so doing, the problems described above are avoided.
However, the PI term corresponds to the correction amount required to eliminate the air-fuel ratio deviation, and therefore, when a restriction is applied to the PI term, the correction amount becomes insufficient such that the air-fuel ratio deviation remains without being eliminated. When feedback control is performed normally with no restriction on the PI term, the PI term gradually converges on a value of a correction amount (referred to as an engine requirement value hereafter) actually required by the engine as the air-fuel ratio deviation is eliminated. When a steady-state error included in the P term is shifted to the I term, the P term converges on zero and the I term converges on the engine requirement value. When the air-fuel ratio deviation remains, however, the air-fuel ratio deviation is repeatedly and continuously integrated into the I term. As a result, when the air-fuel ratio deviation takes a positive value, the I term diverges to positive infinity, and when the air-fuel ratio deviation takes a negative value, the I term diverges to negative infinity.
As described above, when the magnitude of the PI term is restricted by a guard, divergence of the I term occurs as a separate problem. Once the I term has diverged, an excessive correction amount is calculated when the operating condition varies such that the guard on the PI term is removed. Further, when correction of the operating amount through feedback control is resumed following removal of the guard on the PI term, the I term starts to converge toward the engine requirement value again, but after the I team has diverged, it takes a long time for the I term to converge on the engine requirement value.
Divergence of the I term occurring when the PI term is restricted by a guard can be prevented using following methods, for example. In a first method, updating of the I term is stopped while the value of the PI term exceeds the value of the guard. While updating is stopped, the value of the I term is held at a value immediately before updating was stopped. According to this method, divergence of the I term can be prevented reliably. However, convergence of the I term on the engine requirement value is delayed by an amount corresponding to the time during which updating of the I term is stopped.
In a second method for preventing divergence of the I term, as disclosed in Japanese Patent Application Publication No. 2010-249000 (JP 2010-249000 A) and Japanese Patent Application Publication No. 2004-060613 (JP 2004-060613 A), for example, a guard that restricts the magnitude of the I term is provided. According to this method, divergence of the I term is stopped by the guard, and therefore a situation in which the I term diverges to infinity or negative infinity is prevented. In this case, when correction of the operation amount through feedback control is resumed after removing the guard on the PI term, the I term varies toward the engine requirement value using the value of the guard as a departure point. However, the value of the I term guard is set at a considerably larger value than a normally envisaged engine requirement value, and it therefore takes a long time for the I term to converge on the engine requirement value.
The two methods described above are effective methods for preventing divergence of the I term in a condition where the PI term is restricted by a guard. As described above, however, these methods are problematic in terms of a convergence property of the I term following removal of the restriction applied to the PI term by the guard.
Patent Document 1: Japanese Patent Application Publication No. 2008-291752
Patent Document 2: Japanese Patent Application Publication No. 2010-249000
Patent Document 3: Japanese Patent Application Publication No. 2004-060613